1. Field of the Invention
The present invention pertains to the field of electromagnetic (EM) emissions. More particularly, this invention relates to reducing peak EM emissions from a ground plane and a power plane of a printed circuit board (PCB).
2. Background
A wide variety of electrical components have become part of everyday life. In a typical day, a person may use an alarm clock, a computer, a television, and countless other electric powered tools and gadgets. The electric current used to operate each of these components produces both electric fields and magnetic fields, collectively referred to herein as electromagnetic (EM) fields.
As current passes through a conductor, the conductor acts like an antenna to transmit EM fields into the surrounding environment, much like a radio transmission antenna. Conversely, when a conductor is placed in an EM field, current is generated in the conductor, much like a radio reception antenna. So, the EM fields emitted by one electric component can induce electric current, or EM interference (EMI), in another electric component. If the amplitude of the EMI is high enough, it may disrupt normal operations. In order to ensure that electrical components can operate in close proximity to one another, various governing bodies, such as the Federal Communications Commission (FCC) in the United States, impose restrictions on EMI.
Printed circuit boards (PCBs), to which integrated circuits are coupled, can generate relatively large EM emissions. For instance, a power plane and a ground plane in a PCB act like antennas. For each clock cycle of an integrated circuit, numerous transistors may simultaneously draw current off the power plane or drive current onto the ground plane. These simultaneous current fluctuations from the power plane or onto the ground plane can cumulatively create large amplitude EM emissions.
As transistor densities increase for integrated circuits, the EMI restrictions become increasingly difficult to meet. Various cost-added techniques, such as noise filtering, can reduce EM emissions from PCBs. The large copper planes, however, and ground planes in particular, are very difficult to filter effectively, making power and ground planes large contributors to EM emissions.
Therefore, a need exists for a method and apparatus to reduce peak EM emissions from power and ground planes.